1. Field
The system and method described in this patent specification are in the field of electric machines such as motors and generators and pertain more specifically to enabling such machines to operate at steady-state with a selective difference in angular velocity between the mechanical rotation of the rotor (armature) and the magnetic field it generates. Stated differently, at steady-state the rotation of the rotor's magnetic field (flux) is synchronized but the rotor's mechanical speed is selectively different so as to obviate the need for torque gearing when it is desired to have the rotor's mechanical speed different from the synchronization speed.
One example is the ability to drive an electric generator with a prime mover such as a turbine that works more efficiently at speeds higher than the typical synchronization speed of a power generator (e.g., at a speed higher than 3000 or 3600 rpm for electrical power at 50 or 60 Hz, respectively), without using torque-transmitting gearing between the prime mover and the generator.
2. Background
Electric motors and generators have been used for over a century, the principle dating back to Faraday and Fouquet. This principle can be simply stated: a current traveling along a wire in a magnetic field generates a force which pushes the wire relative to the field and thus converts electric power to mechanical force and, conversely, if a wire in a magnetic field is moved relative to the field by a mechanical force (greater than the electromagnetic force), a current in the wire or a voltage across the wire is generated and thus mechanical motion is converted to electric power. To satisfy the various requirements of energy generation standards, different forms of electric generator/motor systems have been devised. Broadly categorized, the electric machines serving as generators or motors are: (1) direct current (or DC) (2) alternating current (or AC) and (3) induction type. All of them can be said to obey the electromotive principle of Faraday, as later more precisely described by Lenz, to the effect that an electric current-carrying wire subject to a magnetic field should produce a resultant force perpendicular to both the current and the magnetic flux.
A typical DC machine has a stator creating a stator magnetic field (flux) that is essentially stationary in space (or at least relative to the rotational axis of a rotor), and a rotor carrying armature windings terminated by commutator segments. Brushes make electrical contact with the commutator segments to carry electric power to the rotor armature windings and thereby generate a rotor magnetic field in the armature, the axis angle of which forms an angle with that of the stator field, producing electromotive force. After the armature has rotated through a certain angle, a new set of commutator segments makes electrical contact with the brushes to thereby continue to provide electromotive force. When at steady-state the armature is rotating in a fixed stator magnetic field, the magnetic field generated by the armature also is approximately fixed in space (moving only through an angle related to that of the commutator segments), but relative to the armature windings the field generated by the armature is rotating in the reverse direction at approximately the same angular speed as the rotor's mechanical speed. If the machine is operating as an electric motor, the higher the current through the armature windings the more torque it will generate. On the other hand, since the windings rotate in the fixed stator field, the armature winding should generate electric power too. The faster the rotation the higher is the power generated in this manner in the armature windings. This power ("back e.m.f.") counters the electric power applied through the commutators to reduce the current flow in the armature. This in effect prevents a typical DC motor from reaching run away speed r.p.m. (revolutions per minute). The combination of the stator magnetic field and armature field distorts the axis of the magnetic flux. Some higher quality DC motors can shift the brush position according to the rotating speed to improve efficiency and thus the output of the motor. This means that the brush position controls the axial direction of the armature field. Similarly, in the generating mode the brush sometimes is rotated according to the generator speed in order to improve electric output.
A typical AC generator/motor operates in a time varying field in that the field changes direction in space with time. If the commutator of a DC generator is exchanged for a pair of electric conducting rings with a pair of brushes in contact with them (to extract power from the rotating armature), then that becomes a typical AC generator.
If one reverses the position of the armature and the field magnetic field, the electric brushes can be eliminated. The power output is proportional to the magnetic field strength and the frequency is a direct function of rotating speed. This inversion is the basis of the design of a modern synchronized electric generator. The rotating magnetic field is fed DC current from a pair of slip rings and brushes. The adjustment of this current controls the electric power output. The advantage of the synchronized generator is that the rotating field does not cut across a designed-in fixed magnetic flux and therefore little or no back e.m.f. is generated. The system is further refined such that the current can be fed through a small AC generator rectified into DC current. The strength of the small AC generator is regulating its DC field strength. The small AC to DC generator can be mounted on the same shaft as the armature and the output is permanently connected to the armature feeds, thus eliminating the slip ring and the brushes. In the motor operating mode, a rotating field has to be provided--a convenient way of doing this is by using a 3-phase AC power with delta- or Y-windings. That AC machine rotates the magnetic field while the armature is in DC mode; in contrast to the DC machine which has the magnetic field in DC mode (or fixed) and the field rotating backwards in the armature.
An induction motor/generator has a rotating electric field that can be produced either by a 3-phase AC power source or by a single phase AC, but retarding a corner of the field by an inductive winding shorting to itself. A desirable simplicity of the motor is in the fact that the armature winding can be in the shape of squirrel cage. The current in the armature is induced or produced by the difference in rotating speed between the field and the armature. That difference is called the slip speed. If the induction motor is operating at synchronized speed, the conductors in the winding have no substantial interaction with the magnetic field and so no induced current to provide an operating force. Hence the induction motor has to be retarded relative to the field rotating speed or accelerated beyond it. Therefore, the induction machine is a mixed mode machine. It has the advantage of simplicity but can not be used conveniently as a constant speed or fixed frequency machine.
An AC synchronized machine links its rotating speed to line frequency so that the most a 60 Hz machine can rotate is at 3600 r.p.m., and a 50 Hz machine at 3000 r.p.m., or an integer fraction of that speed. This can create difficulties for the power generation industry and the machine designer in that the prime mover that drives a generator has to be operated at the synchronized speed also (except the induction generator which has to be given a reference frequency). With the advent of prime mover improvements, some prime mover engines can produce very high horse power if allowed to operate at very high r.p.m. This means one can reduce the engine weight only to have to add on a heavy, torque gear box, which would increase maintenance and cost.